I am looking at using http://drupal.org/project/commons , but I am not sure how easy it is to update a distribution when a new version comes out.
Also, can I use drush to do it, or do I have to manually download and unpack the distribution?
I've always done it the same way as updating Drupal. Download, delete all the old files/folders, and upload the new. Some distributions (Commerce Kickstart) moves everything inside the distribution so you can't just go deleting willy nilly on some. Not sure about Commons but you can check and make sure you're not deleting anything needed. Some distribution maintainers are nice and also provide patches to update only what is necessary.
With drush for commons 7.x you can do:
drush up drupal -y drush dl commons -y drush updatedb -y